More details would be helpful. What is the final product you wish to produce? What do you mean you can't resize your photos? What steps are you following? What happens when you try? Some of us don't know what the "Costco program" is.
I have group shots from my family reunion. In my files I attempted to resize the finished shots using 5 X 7. When I upload the shots to Costco to have them printed, their program requires cropping. Their method of cropping is cutting the photo where I lose some of the scene. I need to know how to shrink the picture to fit a 5 X 7 finished product.
Sorry I was not clear.
It is useful, but it seems like you have misunderstood its purpose.
If you have a photo that is, for example, 3000x2000 pixels, you could resize it smaller for e-mail without changing the aspect ratio, so for example it is now 600x400. No distortion happens in this case, and the photo is now smaller and a more appropriate size for e-mail. That is the purpose of the resize function.
You could also resize it to a different aspect ratio, say 2100x1500, which is a 5x7 aspect ratio (actually 7x5), but this distorts the image and most photographers don't like that.